The present invention relates generally to DC electrical systems, and more particularly, to a method and apparatus for enhancing an arc fault signal for detection in a photovoltaic (PV) system.
The US and other countries have been experiencing record numbers of PV installations in recent years. In one recent year, for instance, the US experienced 339 MW of grid-connected PV during the first 6 months of the year, which represents a 55% increase over the 435 MW that was installed in the entire previous year. Not only has the number of systems increased dramatically in recent years, but the number of large scale systems has increased as well.
Generally, as known in the art, a PV system includes individual solar modules that are connected in series to form a string of, typically, 8-12 modules. A group of strings are connected in parallel in a combiner box, which typically includes a fuse for each positive string wire, and the fuse(s) feed a positive bus bar. Negative wires are also collected within the combiner box to form a negative bus. Conductors sized to handle the combined current and voltage produced at the combiner boxes carry DC power to a master combiner (which may also be regarded as an array combiner or a re-combiner), where combiner box outputs are combined in parallel. Output from one or more master combiners travels through large conductors to a central inverter, and DC power from the master combiner is output as AC power from the inverter. The inverter output is fed to a transformer that converts the output AC voltage to the utility's transmission voltage.
PV systems are expected to be highly robust and reliable for at least twenty years of operation. However, like many high voltage electrical systems, PV systems are susceptible to failure due to, among other things, arcing that can occur in the system. Arcing is a luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of electrodes. An arc fault is an unintentional arcing condition in an electrical circuit and can be caused by, for instance, worn conductor insulation, exposed ends between broken conductors, faulty electrical connections, or loose connections where conducting elements are in close proximity to each other, as examples. The plasma formed during an arcing fault can reach temperatures in excess of 5000 degree C. in a very localized area. This heating can be sufficient to melt surrounding components that are made of plastic or metal, such as fuse holder, parts of disconnect switches, and even the combiner box enclosure itself. This can lead to injury, equipment and property damage, and fires due to ignition of building or PV materials, threatening the loss of building contents and occupant safety.
PV systems are at risk of developing a fault due to the very large number of connections in the system. Thousands of connections can exist in a PV system, giving thousands of opportunities for poor connections to develop over time. A large PV system can have over one hundred combiner boxes, as an example. Thus, there can be thousands or even many thousands of opportunities for faults to occur. Bus bar connections are typically bolted together, and there can be any number of these bolted structures within each combiner box. Within a combiner box, field terminated strings and bus feed wiring particularly have a high potential for developing loose connections, and bus bars and associated termination hardware also have the potential to become loose through electrical and thermal cycling. The risk of developing a fault is even higher for PV strings where the signal needs to propagate down the PV wiring, through PV connectors, as well as through PV module ribbon and cell interconnections.
More so, PV systems are particularly at risk because of damage from sun, wind and weather that can occur over system working life and from the conditions that occur where PV systems are typically installed. That is, the relatively harsh conditions on building roofs, in open fields, etc. . . . can lead to physical damage and accelerated aging of the PV system. Exposure to wind, harsh winter cold and extreme summer heat can weaken connections anywhere throughout the system, causing loose connections. And, because of the dramatic growth in the number of deployed PV systems in recent years, the risk of fire and other damage has only increased. In fact, PV fires have been reported in recent years that have been traced back to component overheating and arcing, particularly caused by loose connections.
Until recently and before the dramatic growth in the number and sizes of PV systems, the risk of fire has been relatively low. And, because of the various types of possible failure modes of PV systems, protective devices employed have generally been directed toward the types of failure modes that are most easily and cost-effectively addressed. Thus, some of the early types of protection employed in PV systems has been in the strings of modules and in the combiner boxes themselves. Known practices thus included placement of an overcurrent protective device (i.e., a fuse) at the load end of a string, as an example. Known combiner boxes as well have included fuses on one or both of the positive and negative conductors that are coupled to the strings. In the event of a short, the fuses placed in such known systems provides a base level of protection against fire, and significant portions of the array connections are provided this base level of protection.
However, placing fuses within the strings and the combiner boxes does not detect arc faults or stop strings or string arrays from generating energy under an in-circuit (e.g., series) arc fault or most short circuit (e.g., parallel) arc faults which can result in fire. Fuses at the load end of a string do not prevent such a fault.
As such, more sophisticated arc fault protection devices have been developed that detect an arc and interrupt the flow of energy before a fire is able to occur. One known device known as an arc fault circuit interrupter (AFCI) includes a number of separable contacts that are responsive to the arc fault detector. Typically, in a DC electrical circuit such as a PV system, a base line of electrical noise exists in the DC power feed (e.g., DC supply or DC return). However, when an arc occurs, a broadband high frequency electrical signal is generated due to the changing nature of the arc plasma. In other words, when an arc is initiated it manifests itself as a broadband AC noise generated on an electrical conductor that can propagate along the PV electrical circuit. As such, in this known device, by selective placement of high frequency detection circuits and selective placement of the separable contacts, the risk of fire can be significantly reduced because the arc can be detected in the very early stages of development.
However, because of the types of electrical devices in a PV circuit, the detectable high frequency components associated with an arc can become attenuated and may not be detectable, at least in the early stages of arc fault development. Typically, for instance, the capacitance within an inverter/load provides low impedance for the high frequency (HF) current generated by the arc fault. DC/DC converters, transformers, inductors, EMI filters, and the like that are between the PV array (power source) and the inverter (load) can cause the HF current to attenuate or not propagate. That is, between where the arc fault occurs and where the detection occurs, and because of the multiple electrical paths within a PV system, the high frequency signal generated from the arc fault can attenuate or not propagate and be undetectable. Thus, in this known system, although a significant degree of protection against arc faults can be provided, it is possible that attenuation within the PV system precludes detection at some AFD locations during the development of the arc fault and a fire can nevertheless develop.
As such, it would be desirable to have a system and method capable of enhancing detection of broadband high frequency components generated in an arc fault within a DC electrical system and particularly within a PV system.